1. Field of the Invention
This invention relates to the field of electric power measuring devices. More specifically, this invention relates to the accumulating and recording of power factor and volt-amperes on single-phase and poly-phase AC power systems.
2. Description of the Prior Art
Electric power is ordinarily delivered to residences, commercial facilities, and industrial facilities as an Alternating Current (AC) voltage that approximates a sine wave with respect to time, and ordinarily flows through the facility as an AC current that also approximates a sine wave with respect to time. Ordinarily, a watt-hour meter is used to charge for the power that is consumed.
It is possible for an AC load to require large amounts of current for long periods of time at standard voltage, and still consume very few watt-hours. For example, a pure inductive load consumes no watt-hours but still demands current from the distribution system. To fairly allocate the costs of their system under this situation, most providers of electric power also measure volt-ampere-hours, directly or indirectly. The ratio of watt-hours to volt-ampere-hours is known as power factor, and is often used to adjust the economic charge for watt-hours.
Traditional power factor measurements relied on measuring the phase-shift between voltage and current, directly or indirectly. These measurements were accurate when both the voltage and the current were sinusoidal with respect to time. However, an increasing proportion of the industrial and commercial load base is made up of loads that draw non-sinusoidal current (for example, most adjustable speed drives, arc furnaces, and data processing systems). More recent power factor measuring instruments measure the ratio of watt-hours to volt-ampere-hours, and are accurate regardless of the waveform of the voltage or the current.
In an AC power distribution system, the expected fundamental frequency of voltage or current (usually 50 Hertz, 60 Hertz, or 400 Hertz) is usually referred to as the fundamental, regardless of the actual spectral amplitude peak. Integer multiples of this "fundamental" are usually referred to as harmonic frequencies, and spectral amplitude peaks at frequencies below the fundamental are often referred to as "sub-harmonics", regardless of their ratio relationship to the fundamental.
It is widely recognized that loads which draw harmonic currents place an increased economic burden on the power distribution system by requiring derated transformers, increased conductor area, installation of filters, and other actions. If the non-fundamental currents--harmonics or sub-harmonics--are large, relative to the impedance of the distribution system, they can induce harmonic voltages in the voltage delivered to other loads that share the distribution system.
Under these circumstances, even power factor measurements fail to accurately measure the economic impact of the load. The economic impact of such a consumer can negatively impact other users or the distribution system.
For example, on a system designed for 60 Hz power transmission and distribution, an accurate power factor measurement treats 100 amperes at 60 Hertz in exactly the same way as 100 amperes at 540 Hertz, although the latter almost certainly requires a larger investment by the power provider in transformer rating and conductor size.
Harmonic adjustments to power factor measurements provide a better estimate of the economic impact of a non-linear load.